Combine flour, water, and 2 1/2 teaspoons salt in a stand mixer and mix on low until combined, about 4 minutes. Rest for 20 minutes.

Sprinkle yeast and sugar over dough. Knead on low for 2 minutes, then on high until glossy and smooth, 8 to 10 minutes.

Coat a bowl with 1 tablespoon oil, transfer dough to the bowl, coat with another 1 tablespoon oil, cover, and let rise for 2 to 2 1/2 hours.

Heat 1 tablespoon oil in a skillet over medium heat. Add onions and 1 teaspoon salt and cook until golden brown, about 10 minutes. Remove from heat and stir in poppy seeds.

Coat a baking sheet with 1 tablespoon oil. Turn dough out onto the prepared sheet.

Press dough out to edges, let rise uncovered for 30 minutes.

Press dough out again. Poke surface with a fork 30 to 40 times. Brush with 1 tablespoon oil and sprinkle with 1 1/2 teaspoons salt. Distribute onion-poppy seed mixture evenly.

Bake at 500 degrees until golden brown, 18 to 23 minutes, rotating halfway through. Transfer to a cutting board, slice, and serve.